1. Field of Invention
The present invention relates to compounds which act as immunosuppressive medicinal agents to reduce a high population of circulating leucocytes in animals and alleviate untoward effects produced thereby. More specifically, the 2-acylimidazole compounds which are synthesized and isolated according to the process of the present invention are useful to promote tissue transplant reception, and treat cell-mediated autoimmune disease.
Leucocytes, or white blood cells (WBC's), are blood cells which do not contain hemoglobin, and are phagocytic and immune responsive in nature. WBC's can be classified into five major groups, one of which is the lymphocyte group. A major function of the WBC's is to defend the body against invasion by foreign biological material such as bacteria and viruses. A major function of the lymphocytes, in particular, is concerned with the immune response which is the body's recognition and removal mechanism for handling foreign substances.
2. Description of the Prior Art
Depression of circulating leucocyte populations in animals has been discussed extensively in the scientific literature.
For example, in "The Chemistry and Metabolism of 4'-Deoxypyridoxine," CRC Press (1981), Library of Congress No. QP772.P9C6 Stephen P. Coburn reports that 4'-deoxypyridoxine has hematologic effects. When this compound is administered to rats at doses as low as 1 mg/kg and in conjunction with a vitamin B.sub.6 -deficient diet, it will produce leucopenia with lymphopenia and increased neutrophils. Atrophy of lymphoid elements in the thymus is consistently observed. After restoration of a normal diet, however, circulating lymphocytes double within two days and the cell population returns to normal within two weeks. When deoxypyridoxine is withdrawn but the deficient diet continued, the leukocyte count returns to normal but the granulocyte to lymphocyte ratio does not.
Coburn also has found that deoxypyridoxine causes a reduced vitamin B.sub.6 uptake in human leucocytes and platelets. For cultured leucocytes treated with this compound, the maximum cellular concentration of pyridoxal phosphate (B.sub.6) is about 37 ng per 10.sup.8 cells while the normal level is significantly higher. Deoxypyridoxine also causes a detectable decrease (ca. 15%) in the assimilation of the B.sub.6 precursor pyridoxine into the cells even at a 1 to 1 ratio of deoxypyridoxine to pyridoxine. Similar effects on erythrocytes are also reported for a 1 to 1 ratio of deoxypyridoxine to pyridoxine.
The immune system seems to be sensitive to B.sub.6 deficiency, and deoxypyidoxine appears to exhibit a cooperative effect with this aspect of B.sub.6 deficiency. For example, it is known that the lymphoid system and consequently the immune system are influenced adversely by reducing or by eliminating specific nutrients and enzymatic cofactors from the diet. "Human Vitamin B.sub.6 Requirements," National Academy of Sciences (1978). Several studies have shown that a dietary deficiency of vitamin B.sub.6 may result in an impairment of both humoral and cell-mediated immune responses.
The Coburn studies confirm cooperation of vitamin B.sub.6 and deoxypyridoxine to affect the immune system. When rats are given deoxypyridoxine (0.1 mg/ml drinking water) in combination with a B.sub.6 -deficient diet, their total leucocyte population is reduced. Their neutrophils are somewhat increased while their lymphocytes are substantially reduced. Moreover, mixed lymphocyte response as measured by thymidine uptake is reduced 55 percent, the normal lymphocyte transfer reaction is reduced, and uridine incorporation into small lymphocytes is decreased.
Deoxypyridoxine also affects the immune response of primates, as shown by the Coburn report of paralytic poliomyelitis in an unvaccinated rhesus monkey (a very rare occurrence). For 17 days previous to exposure to poliomyelitis virus, the monkey received subcutaneous injections of 100 mg deoxypyridoxine per kilogram body weight (twice daily during the week, once on weekends) along with a B.sub.6 deficient diet. Viral exposure then produced paralysis. Coburn suggests that suppression of the immune system, which would cause conditions favoring increased virulence of the virus, possibly produced the paralysis.
Reduced leucocyte populations may also be helpful for tissue transplantation. Coburn cites studies of the effect of B.sub.6 deficiency on tissue transplantation. These studies show that B.sub.6 deficiency either alone or combined with intraperitoneal injection of 250 or 500 mg deoxypyridoxine per day increase the survival of skin grafts in rats. Coburn concludes that the deoxypyridoxine may have potential clinical application in tissue transplantation.
Cell-mediated autoimmune disease is a debilitating condition which results when the immune system attacks normal, endogeneous tissue rather than foreign vectors. Often, substantial populations of leucocytes are found in such affected tissues and concurrent immune responses such as histamine release, lymph fluid infiltration, and leucocye migration are also present. Destruction of the affected tissue eventually occurs. The inability of the immune system to distinguish foreign from endogeneous tissue may be based, at least in part, upon stimulated cell growth and concurrent cell genial selection. In turn, it is felt that decreasing such cell populations, especially of immune cells which trigger phagocytosis, would modify the course of autoimmune disease. Accordingly, pharmaceutical agents which depress leucocyte populations may be helpful in the treatment of autoimmune disease.
Deoxypyridoxine can be seen to be useful for the foregoing purposes. However, it produces a further decrease in vitamin B.sub.6 deficiency when it is administered to animals receiving a B.sub.6 deficient diet. This is undesirable since control of the B.sub.6 level in such therapy should be possible.
Accordingly, it is an object of the invention to develop an immunosuppressive medicinal agent which does not adversely effect vitamin B.sub.6 levels. Another object is the development of a highly active agent which does not necessitate administration of large doses in order to achieve the desired therapeutic effects.